The present invention relates to a method for aligning a cutter spindle of the granulator and in particular a submerged granulator relative to a cutting plate. The invention also relates to a granulator and in particular to a submerged granulator for producing plastic granulate, which has a cutting box and a cutter spindle, on which a cutter holder is supported which is rotatably received in the cutting box, and a drive motor connectable with the cutter spindle, wherein during operation the cutter holder cooperates with a cutting plate.
Granulators, in particular submerged granulators, are used for producing plastic granulate. Plasticized thermoplastic material is hereby pressed through a cutting plate having suitable openings and is stripped or cut off by cutters moving past the openings, wherein the cutters are typically held by a cutter holder. In a submerged granulator, the produced granulate in form of short plastic strands is cooled by a water stream which simultaneously transports it away from the cutting region. The cutting process is typically performed by rotating cutters which rotate in front of openings disposed in a cutting plate. The cutter holder holding the cutters rotates in front of the cutting plate. The openings are usually substantially circular and disposed in a cutting region, across which the cutters move, in the preferably heated cutting plate.
The quality of the granulate produced in this manner must meet high quality standards. In particular, the produced granulate should be uniform and pourable. It is particular important for attaining a high-quality of the produced granulate that the distance between the cutters to the surface of the cutting plate having the opening is as small and as uniform as possible. The greater the distance, the more frequently the granulates can produce fuzz or fibers, i.e., lacerated edge regions which would hamper a good pourability.
To provide both a granulate of good quality as well as a high reliability of the process, it must be ensured that the distance between the cutter and the surface of the cutting plate is as small and as uniform as possible. The cutter spindle which rotatably supports the cutter holder must have the most advantageous and essentially perpendicular alignment with respect to the surface of the cutting plate.
These requirements become even more important with increasing facility size and hence also the increase in the diameter of the circular holes in the cutting plate. Poor alignment can quickly cause at least in certain regions an overly large cutting gap. For example, with poor alignment the cutters may contact the cutting plate, for example, in a region of the circumference of the traversed arc, which may cause uneven wear, whereas a larger gap is created in another region, resulting in poor quality of the granulate. In addition, in particular with high throughput, each idle time represents a significant economic loss, because each facility which processes, for example, 50 t/h, produces daily about 1,200,000 kg polymer granulate. Such granulate facility includes generally at least one extruder, at least one granulator, and downstream devices for further processing of the granulate.
Until now, the cutter spindle position was measured with the granulator at rest and manually corrected through adjustment of support threads. For example, one side of the cutting chamber, in which the cutter holder including the cutters is rotatably received in front of the cutting plate, was opened to correct the alignment. The measurement accuracy is here in a range of 0.01 mm, so that the alignment accuracy is also in that order of magnitude, but maybe generally greater, for example in a range of 0.03 mm.
However, this alignment is not performed during operation, but when the granulator is at rest. However, disturbances occur during operation, for example extreme temperature differences between the cutting plate, which can have a temperature in excess of 200° C., and the cutting space, which must have a temperature so that the produced granulate quickly cools down and solidifies. In particular, with submerged granulators, where water flows through the cutting space, the temperatures in the cutting space are frequently less than 50° C. In addition, the water pressure also affects the alignment. Due to these disturbances, which may cause, for example, elastic deformations and differences in the thermal expansion of individual components of more than 0.01 mm, the adjustment accuracy is additionally reduced. Keeping in mind that the maximally allowed cutting gap, where adequate quality can be assured, is about 0.1 mm, it can be readily seen that it is very difficult to maintain a constant cutting gap across the entire cutting plate.
To solve this problem and to guarantee a constant end particularly minimum cutting gap, a gimbal mount of the cutting head with constant contact pressure is frequently employed. This means that there is contact between the cutters and the cutting plate and a contact pressure is constantly applied, causing premature wear of the cutters.